1. Field of the Invention
The present invention relates generally to the field of ocular surgery, and more specifically to a method and apparatus for controlling a phacoemulsification handpiece and needle configured for operation at multiple ultrasonic frequencies during ophthalmic surgical procedures.
2. Description of the Related Art
Phacoemulsification surgery has been successfully employed in the treatment of certain ocular problems, such as cataract surgery, including removal of a cataract-damaged lens and implantation of an intraocular lens. Phacoemulsification surgery typically involves removal of the cataract-damaged lens and may utilize a small incision at the edge of the cornea. Through the small incision, the surgeon then creates an opening in the capsule, i.e. membrane that encapsulates the lens.
The surgeon can insert an ultrasonic probe, incorporated within a phacoemulsification handpiece, through the opening in the cornea and capsule accessing the damaged lens. The handpiece's ultrasonically actuated tip emulsifies the damaged lens sufficient to be evacuated by the handpiece. After the damaged natural lens is completely removed, the handpiece tip is withdrawn from the eye. The surgeon may now implant an intraocular lens into the space made available in the capsule.
While performing phacoemulsification surgical techniques, such as lens removal, the surgeon may desire to employ either a longitudinal motion or a mix of longitudinal with transversal motions, also referred to in the industry as transverse phacoemulsification, to affect different desired cutting movements. Certain previously available ultrasonic probe designs provided for only one type of cutting movement such that if the surgeon determines during the procedure a need to switch from, for example, longitudinal to transverse cutting movements, the surgeon has been required to change the ultrasonic probe handpiece. This changing of the handpiece during surgery can complicate and lengthen the procedure.
It is desirable to provide the surgeon with a single handpiece capable of operating in either the longitudinal or transverse modes or some mix of them that provides an elliptical mode. The frequency of handpiece operation for longitudinal or transverse modes are determined by the physical and electrical properties of an ultrasonic handpiece. The operational frequencies are on or about the resonant frequencies of the handpiece and these frequencies are generally different for longitudinal and transverse modes. The driver of such a handpiece must therefore operate at multiple frequencies.
Past handpiece drivers were linear class AB types or similar. Although these drivers exhibited a flat (acceptable) frequency response and were therefore capable of handling a multiple frequency handpiece, they were very inefficient in power delivery, required heatsinking, had a high quiescent current, suffered from crossover distortion, and were generally complex designs. Power dissipation in these drivers is high and the resulting heat can be destructive if not properly managed.
Modern handpiece drivers are class D designs. A class D driver is operated in an ON/OFF mode or Pulse Width Modulation mode instead of linear mode. This type of driver is very efficient and less complex than a class AB driver. Because of the digital (ON/OFF) nature of the driver, the input signal is digital in nature (pulse width modulation, pulse frequency modulation, etc) and the driver output is also digital in nature. When used to drive an ultrasonic handpiece on or about its resonant frequency, it is desirable to remove all but the fundamental frequency from the driver output so as to limit power dissipation in the handpiece at undesirable frequencies and to eliminate distortion in handpiece voltage and current feedback signals that are used to control handpiece operation. Consequently, a filter circuit is generally inserted between the driver output and the handpiece to remove unwanted harmonics and provide a sine wave drive signal to the handpiece.
Since the driver output is generally a step-up transformer to generate the high voltages required to drive the handpiece and because of filter power dissipation, the filter circuit is generally made of passive components (L, R, C). 2-Pole L-C filters are preferred for lower dissipation and fast roll off of gain with frequency but they suffer from gain peaking around the cutoff frequency.
Typical current designs employ a single fixed output filter for a needle operating at multiple frequencies, primarily to minimize costs and complexity. However, use of a single output filter design can result in undesirable third harmonic frequencies to the handpiece when the phacoemulsification handpiece operates at more than one frequency, particularly when switching between frequencies. Variations in gain experienced at different frequencies may limit the ability to deliver sufficient power at each frequency to adequately drive the handpiece needle.
Single output filters are limited to providing only one fixed relatively low cut-off frequency. An output filter designed to provide gain at the fundamental frequency and rejection of third and higher harmonics for a 38 kHz ultrasonic handpiece cannot maintain a comparable or consistent gain and third harmonic rejection for a 26 kHz ultrasonic handpiece using simple passive filtering.
If the low cut-off frequency for the output circuit is set at too high a level for use with a 38 kHz handpiece a relatively significant third harmonic energy will occur in the output signal resulting in a distorted waveform when driving a 26 kHz handpiece. When a waveform distorts in this manner it becomes difficult for system monitoring circuits to measure the correct voltage and current values used to excite the piezoelectric crystal and to control handpiece operation.
When configuring a single output filter circuit to provide a sufficiently low cut-off frequency for removing the third harmonic distortion from the output signal waveform, resulting from operating the circuit at the first lower frequency such as 26 kHz, the circuit's resultant gain at 38 kHz may be too high or too low depending on placement of the cutoff frequency and the Q of the L-C filter.
Based on the foregoing, it would be beneficial to offer a single output filtering mechanism design for operating an ultrasonic probe at multiple frequencies that overcomes the foregoing drawbacks present in previously known designs used in the ocular surgical environment.